THough Handel C, SystemC, SpecC are not true HDLs!
Handle C is High-level language based on ISO/ANSI-C for the implementation of algorithms in hardware
• Allows software engineers to design hardware without retraining
• Clean extensions for hardware design including flexible data widths, parallelism and communications
• Core building block for Celoxica™ DK1 design suite
systemC is more popular in industry and supported by mainstream EDA vendors.
There're emerging languages these years, systemC, systemVerilog, Handle-C, Vera, E language and etc., I think there'll be kinda cleanup later, after which only 1 or 2 will survive.
from my limited experience, Handle-C and system C are better suited to modelling, and creating coarse firmware solutions. They are no where near as effecient as well crafted HDL.